.250 O'Neil Magnum
The .250 O'Neil Magnum was a wildcat cartridge created around 1936 by Con Schmitt and Charles M. O'Neil, the latter being the cartridge's namesake. It was intended to compete with a notoriously accurate German cartridge, the .244 Halger. It was also meant to match the .244 Halger's high muzzle velocity. The concept went through many iterations with various cartridges being necked down, shoulder angles changed, and so on. This final cartridge was created by necking down the .275 H&H Magnum to .25 caliber.